"Offhand, I can only think of Columbine and Jonesboro, Arkansas," Mike Roche, a retired Secret Service agent and former Arkansas police officer, told ABC News of cases of multiple shooters.

"Most of the multiple sightings are attributed to eyewitness accounts providing conflicting descriptions of the suspect or the perception of a fleeing person as a potential attacker," said Roche, who's written a book about how to identify mass killers.

"It's just the pandemonium in a stressful situation like that," Roche said.

Science might even be to blame, confusing people about what they've really witnessed. During stressful situations, "cortisol gets dumped into the body and can impact your perception of memory and distort what you see," Roche said.

"We've heard perhaps the gunshots, the screaming, and we see someone running toward us and we capture that image and we're telling the police, well, this is what he looked like - when in fact that is someone who was in the same boat as them, trying to flee the situation." he said.

Another expert pointed to security lockdowns as a way students get confused about the number of shooters.

"This situation happens most often because FEMA (Federal Emergency Management Agency)recommends holding students in a lockdown procedure until law enforcement had confirmed there is no second shooter," Hunter Frederick, who owns a crisis management firm, told ABC News. "That, along with lapse of communication during a crisis, can often lead to confusion."